Stacked microvoided light diffuser

ABSTRACT

The invention relates to a light diffuser comprising a polymeric film wherein the film comprises a plurality of layers having void geometry in which the x/y/z size or frequency varies by at least 28% between at least two layers.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to three applications co-filedherewith under Attorney Docket Nos. 83448AEK, 83613AEK, and 83744AEK,the contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The invention relates to a diffuser for specular light. In apreferred form, the invention relates to a back light diffuser for rearprojection liquid crystal display devices.

BACKGROUND OF THE INVENTION

[0003] Optical structures that scatter or diffuse light generallyfunction in one of two ways: (a) as a surface diffuser utilizing surfaceroughness to refract or scatter light in a number of directions; or (b)as a bulk diffuser having flat surfaces and embedded light-scatteringelements.

[0004] A diffuser of the former kind is normally utilized with its roughsurface exposed to air, affording the largest possible difference inindex of refraction between the material of the diffuser and thesurrounding medium and, consequently, the largest angular spread forincident light. However, some prior art light diffusers of this typesuffer from two major drawbacks: a high degree of backscattering and theneed for air contact. Backscattering causes reflection of a significantportion of the light back to the originating source when it shouldproperly pass through the diffuser, lowering the efficiency of theoptical system. The second drawback, the requirement that the roughsurface must be in contact with air to operate properly, may also resultin lower efficiency. If the input and output surfaces of the diffuserare both embedded inside another material, such as an adhesive forexample, the light-dispersing ability of the diffuser may be reduced toan undesirable level.

[0005] In one version of the second type of diffuser, the bulk diffuser,small particles or spheres of a second refractive index are embeddedwithin the primary material of the diffuser. In another version of thebulk diffuser, the refractive index of the material of the diffuservaries across the diffuser body, thus causing light passing through thematerial to be refracted or scattered at different points. Bulkdiffusers also present some practical problems. If a high angular outputdistribution is sought, the diffuser will be generally thicker than asurface diffuser having the same optical scattering power. If howeverthe bulk diffuser is made thin, a desirable property for mostapplications, the scattering ability of the diffuser may be too low.

[0006] Despite the foregoing difficulties, there are applications wherean embedded diffuser may be desirable, where the first type of diffuserwould not be appropriate. For example, a diffuser layer could beembedded between the output polarizer layer and an outer hardcoat layerof a liquid crystal display system to protects the diffuser from damage.Additionally, a diffuser having a thin profile, which will retain wideoptical scattering power when embedded in other materials and have lowoptical backscatter and therefore higher optical efficiencies thanconventional diffusers, would be highly desirable.

[0007] U.S. Pat. No. 6,093,521 describes a photographic membercomprising at least one photosensitive silver halide layer on the top ofsaid member and at least one photosensitive silver halide layer on thebottom of said member, a polymer sheet comprising at least one layer ofvoided polyester polymer and at least one layer comprising nonvoidedpolyester polymer, wherein the imaging member has a percent transmissionof between 38 and 42%. While the voided layer described in U.S. Pat. No.6,093,521 does diffuse back illumination utilized in prior art lightboxes used to illuminate static images, the percent transmission between38 and 42% would not allow a enough light to reach an observers eye fora liquid crystal display. Typically, for liquid crystal display devices,back light diffusers must be capable of transmitting at least 65% andpreferably at least 80% of the light incident on the diffuser.

[0008] In U.S. Pat. No. 6,030,756 (Bourdelais et al), a photographicelement comprises a transparent polymer sheet, at least one layer ofbiaxially oriented polyolefin sheet and at least one image layer,wherein the polymer sheet has a stiffness of between 20 and 100millinewtons, the biaxially oriented polyolefin sheet has a spectraltransmission between 35% and 90%, and the biaxially oriented polyolefinsheet has a reflection density less than 65%. While the photographicelement in U.S. Pat. No. 6,030,756 does separate the front silver halidefrom the back silver halide image, the voided polyolefin layer woulddiffuse too much light creating a dark liquid crystal display image.Further, the addition of white pigment to the sheet causes unacceptablescattering of the back light. The photographic element contains a layerof voids that are generally the same size. They vary in void volume by10% or less.

[0009] In U.S. Pat. No. 5,223,383 photographic elements containingreflective or diffusely transmissive supports are disclosed. While thematerials and methods disclosed in this patent are suitable forreflective photographic products, the % light energy transmission (lessthan 40%) is not suitable for liquid crystal display as % lighttransmission less than 40% would unacceptable reduce the brightness ofthe LC device.

[0010] In U.S. Pat. No. 4,912,333, X-ray intensifying screens utilizemicrovoided polymer layers to create reflective lenslets forimprovements in imaging speed and sharpness. While the materialsdisclosed in U.S. Pat. No. 4,912,333 are transmissive for X-ray energy,the materials have a very low visible light energy transmission which isunacceptable for LC devices.

[0011] In U.S. Pat. No. 6,177,153, oriented polymer film containingpores for expanding the viewing angle of light in a liquid crystaldevice is disclosed. The pores in U.S. Pat. No. 6,177,153 are created bystress fracturing solvent cast polymers during a secondary orientation.The aspect ratio of these materials, while shaping incident light,expanding the viewing angle, do not provide uniform diffusion of lightand would cause uneven lighting of a liquid crystal formed image.Further, the disclosed method for creating voids results in void sizeand void distribution that would not allow for optimization of lightdiffusion and light transmission. In example 1 of this patent, thereported 90% transmission includes wavelengths between 400 and 1500 nmintegrating the visible and invisible wavelengths, but the transmissionat 500 nm is less that 30% of the incident light. Such values areunacceptable for any diffusion film useful for image display, such as aliquid crystal display.

PROBLEM TO BE SOLVED BY THE INVENTION

[0012] There remains a need for an improved light diffusion of imageillumination light sources to provide improved light transmission whilesimultaneously diffusing specular light sources.

SUMMARY OF THE INVENTION

[0013] The invention provides a light diffuser comprising a polymericfilm wherein the film comprises a plurality of layers having voidgeometry in which the x/y/z size or frequency varies by at least 28%between at least two layers. The invention also provides a back lightedimaging media, a liquid crystal display component and device.

ADVANTAGEOUS EFFECT OF THE INVENTION

[0014] The invention provides improved light transmission whilesimultaneously diffusing specular light sources.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 illustrates a cross section voided polymer diffusionmaterial that contains stacked voids suitable for use in a liquidcrystal display device.

[0016]FIG. 2 illustrates a liquid crystal display device with a lightdiffuser that contains stacked voids.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The invention has numerous advantages over prior practices in theart. The invention provides diffusion of specular light sources that arecommonly used in rear projection display devices such as liquid crystaldisplay devices. Further, the invention, while providing diffusion tothe light sources, has a high light transmission rate. A hightransmission rate for light diffusers is particularly important forliquid crystal display devices as a high transmission value allows theliquid crystal display to be brighter or holding the level of brightnessthe same, allows for the power consumption for the back light to bereduces therefore extending the lifetime of battery powered liquidcrystal devices that are common for note book computers. The voidedpolymer layers of the invention can be easily changed to achieve thedesired diffusion and light transmission requirements for many liquidcrystal devices thus allowing the invention materials to be responsiveto the rapidly changing product requirements in the liquid crystaldisplay market.

[0018] Multiple layers of varying void size and frequency is advantagedto a layer a comparable thickness containing one size and frequency ofvoid because the multiple layer film becomes diffuses more efficiently.In a polymeric film comprising layers of voids of decreasing size orfrequency in relation to the light passing through the film, the voidedlayers with large void size or frequency diffuses the light coarsely. Asthe light passes through other layers with smaller voids or lowerfrequency of voids, the light is more finely diffused. This increasesthe diffuse light transmission efficiency as compared to a single filmof comparable thickness with only one size or frequency void. Stackingthe layers in order of increasing or decreasing frequency or size hasthe same effect of providing stepwise diffusion and thus is moreefficient than an a single layer of equal void size or frequency.

[0019] The invention eliminates the need for an air gap between priorart light diffusers that contain a rough surface and the brightnessenhancement films used in liquid crystal display devices. Theelimination of the air gap allows for the diffuser materials to beadhesively adhered to other film components in the liquid crystaldisplay making the unit lighter in weight and lower in cost.

[0020] The invention materials do not contain inorganic particlestypical for prior art voided polymer films that cause unwantedscattering of the back light source and reduce the transmissionefficiency of the liquid crystal display device. Further, the elasticmodulus and scratch resistance of the diffuser is improved over priorart cast coated polymer diffusers rendering a more robust diffuserduring the assembly operation of the liquid crystal device. These andother advantages will be apparent from the detailed description below.

[0021] The term “LCD” mean any rear projection display device thatutilizes liquid crystals to form the image. The term “diffuser” meansany material that is able to diffuse specular light (light with aprimary direction) to a diffuse light (light with random lightdirection). The term “light” means visible light. The term “diffuselight transmission efficiency” means the ratio of % diffuse transmittedlight at 500 nm to % total transmitted light at 500 nm multiplied by afactor of 100. The term “polymeric film” means a film comprisingpolymers. The term “polymer” means homo- and co-polymers. The termmicrobead means polymeric spheres typically synthesized using thelimited coalescence process. These microbead spheres can range in sizefrom 0.2 to 30 micrometers. They are preferably in the range of 0.5 to5.0 micrometers. The term microvoids means pores formed in an orientedpolymeric film during stretching. These pores are initiated by eitherinorganic particles, organic particles, or microbeads. The size of thesevoids is determined by the size of the particle or microbeads used toinitiate the void and by the stretch ratio used to stretch the orientedpolymeric film. The pores can range from 0.6 to 150 um's in machine andcross machine directions of the film. They can range from 0.2 to 30micrometers in height. Preferably the machine and cross machinedirection pore size is in the range of 1.5 to 25 micrometers. Preferablythe height of the pores is in the range of 0.5 to 5.0 micrometers. Theterm substantially circular means indicates a geometrical shape wherethe major axis is no more than two times the minor axis.

[0022] The term “x/y/z size” means the length of the void in the x, y,or z direction. The x/y/z size of the polymer void describes the voidgeometry. The z direction is the direction parallel to the light path.The x and y directions lie in the plane perpendicular to the light path.The term “frequency” means the number voids per unit volume. Forexample, a diffuser sheet has a frequency of 7/cm³.

[0023] The preferred diffuser material of the invention comprises twovoided layers because it is easily manufacturable and provides theadvantages seen in multiple layer film structures.

[0024] Preferrably, the polymeric film comprises at least two voidedlayers and at least one non-voided layer, more preferred is thepolymeric material containing at least two voided layers separated by anon-voided layer. This non-voided layer is preferred because it is themore efficient place for addenda such as a polymer processing aid. Itspreferred place of the non-voided layer between the layers of voidedfilms is because it is advantaged to have diffusion before and after thelayer containing addenda.

[0025] The polymeric film comprises a plurality of layers that areintegral. “Integral” means that the layers were formed together at thetime of manufacture and not assembled from a combination of films. Aplurality of layers that are integral is preferred because it reducesthe number of processing steps and creates film interfaces in opticalcontact. Further, an integral layer provides excellent inter-layeradhesion.

[0026] Preferably, the polymeric film comprises addenda that are addedto a layer in the diffuser film to aid in film processing, or to changecolor of the film.

[0027] Preferrably, the light diffuser with a plurality of voided layersvarying in geometry or frequency improve the diffuse light transmissionefficiency compared to a single voided layer of the same thickness andeither void geometry or frequency by at least 10% at 500 nm. 10%increase in the diffuse light transmission efficiency represents asignificant advantage in the use of the ‘stacked’ microvoided diffusionfilm over a monolayer film in an LCD.

[0028] Substantially circular voids, or voids whose major axis to minoraxis is between 2.0 and 0.5 are preferred as substantially circularvoids have been shown to provide efficient diffusion of light energy andreduce uneven diffusion of light energy. A major axis diameter to minoraxis diameter ratio of less than 2.0 is preferred. A ratio less than 2.0has been shown to provide excellent diffusion of LC light sources.Further, a ratio greater than 3.0 yields voids that are spherical andspherical voids have been shown to provide uneven dispersion of light. Aratio between 1.0 and 1.6 is most preferred as light diffusion and lighttransmission is optimized.

[0029] Preferably, two voided layers in the polymeric film differ infrequency or at least one of the dimensions of the void in the x, y, orz, direction by more than 28%. Two layers in the polymeric film thatdiffer in the dimensions in the x, y, z or in frequency by less than 28%could be accounted for by the natural distribution of bead and voidsize. Deviations in the size of the void initiating beads causedifferences in void frequency and dimension up to 25%.

[0030] It is preferred that two voided layers in the polymeric filmdiffer in frequency or at least one of the dimensions of the void in thex, y, or z, direction by more than 28%, but less than 300%. When thefrequency or one of the dimensions of the voids change more than 300%between layers, scattering efficiency is diminished.

[0031] In another embodiment of the invention, two voided layers in thepolymeric film differ in frequency or at least one of the dimensions ofthe void in the x, y, or z, direction by more than 60%. Deviations inthe size of the void initiating beads may cause differences in voidfrequency and dimension up to 60%.

[0032] Layers in “order of increasing size” means that the light passesthough the voided layer which has the smallest sized voids then thoughthe next layer with larger size voids and through layers with increasingsize voids. Layers in “order of decreasing size” means that the lightpasses though the voided layer which has the largest sized voids thenthough the next layer with smaller size voids and through layers withdecreasing size voids. Layers in “order of increasing frequency” meansthat the light passes though the voided layer that has the lowestfrequency voids then though the next layer with higher frequency voidsand through layers with increasing frequency voids. Layers in “order ofdecreasing frequency” means that the light passes though the voidedlayer that has the highest frequency voids then though the next layerwith lower frequency voids and through layers with decreasing frequencyvoids. By passing through these voided layer orientations, the light isgradually diffuse increasing the diffuse light transmission efficiency.

[0033]FIG. 1 is a cross section of the light diffuser 12 of theinvention containing two voided layers. Light diffuser 12 containspolymer matrix layer 22 comprises small voids 24. Large air voids 28 aredispersed in polymer matrix layer 30. The two voided layers containinterface 26.

[0034]FIG. 2 illustrates a liquid crystal display device with a lightdiffuser with multiple polymer voided layers. Visible light source 18 isilluminated and light is guided into acrylic board 2. Reflector tape 4is used to focus of axis light energy into the acrylic board 2.Reflection tape 6, reflection tape 10 and reflection film 8 are utilizedto keep light energy from exiting the acrylic board in an unwanteddirection. Diffusion film 12 containing with multiple polymer voidedlayers is utilized to diffuse light energy exiting the acrylic board inthe direction perpendicular to the diffusion film. Brightnessenhancement film 14 is utilized to focus the light energy intopolarization 16. The diffusion film 12 containing with multiple polymervoided layers is in optical contact with brightness enhancement film 14.

[0035] Better control and management of the back light are drivingtechnological advances for liquid crystal displays (LCD). LCD screensand other electronic soft display media are back lit primarily withspecular (highly directional) fluorescent tubes. Diffusion films areused to distribute the light evenly across the entire display area andchange the light from specular to diffuse. Light exiting the liquidcrystal section of the display stack leaves as a narrow column and mustbe redispersed. Diffusers are used in this section of the display toselectively spread the light out horizontally for an enhanced viewingangle.

[0036] Diffusion is achieved by light scattering as it passes thoughmaterials with varying indexes of refraction. This scattering produces adiffusing medium for light energy. There is an inverse relationshipbetween transmittance of light and diffusion and the optimum combinationof these two parameters must be found for each application.

[0037] The back diffuser is placed directly in front of the light sourceand is used to even out the light throughout the display by changingspecular light into diffuse light. The diffusion film is made up ofsimple optical structures to broaden the light. Prior art methods fordiffusing LCD back light include layering polymer films with differentindexes of refraction, embossing a pattern onto the film, or coating thefilm with matte resins or beads.

[0038] The role of the front diffuser is to broaden the light coming outof the liquid crystal (LC) with directional selectivity. The light iscompressed into a tight beam to enter the LC for highest efficient andwhen it exits it comes out as a narrow column of light. The diffuseruses optical structures to spread the light selectively. Most companiesform elliptical micro-lens to selectively stretch the light along oneaxis. Elliptically shaped polymers in a polymer matrix and surfacemicro-lenses formed by chemical or physical means achieve thisdirectionality.

[0039] The invention provides a film that scatters the incident lightwhile providing high light transmission. The oriented film of thepresent invention can be produced by using a conventionalfilm-manufacturing facility in high productivity. The invention utilizesvoided thermal plastic layers containing microvoids. Microvoids of airin a polymer matrix are preferred and have been shown to be a veryefficient diffuser of light compared to prior art diffuser materialswhich rely on surface roughness on a polymer sheet to create lightdiffusion for LCD devices. The microvoided layers containing air have alarge index of refraction difference between the air contained in thevoids (n=1) and the polymer matrix (n=1.2 to 1.8). This large index ofrefraction difference provides excellent diffusion and high lighttransmission which allows the LCD image to be brighter and/or the powerrequirements for the light to be reduces thus extending the life of abattery. The preferred diffuse light transmission of the diffusermaterial of the invention are greater than 65% at 500 nm. Diffuser lighttransmission less than 60% at 500 nm does not let a sufficient quantityof light pass through the diffuser, thus making the diffuserinefficient. A more preferred diffuse light transmission of themicrovoided thermoplastic voided layer is greater than 80% at 500 nm. An80% at 500 nm diffuse transmission allows the LC device to improvebattery life and increase screen brightness. The most preferred diffusetransmission of the voided thermoplastic layer is greater than 87% at500 nm. A diffuse transmission of 87% at 500 nm allows diffusion of theback light source and maximizes the brightness of the LC devicesignificant improving the image quality of an LC device for outdoor usewhere the LC screen must compete with natural sunlight.

[0040] Since the microvoids of the invention are substantially air, theindex of refraction of the air containing voids is 1. An index ofrefraction difference between the air void and the thermoplastic matrixis preferably greater than 0.2. An index of refraction differencegreater than 0.2 has been shown to provide excellent diffusion of LCDback light sources and a index of refraction difference of greater than0.2 allows for bulk diffusion in a thin film which allows LCDmanufacturers to reduce the thickness of the LC screen. Thethermoplastic diffusion layer preferably contains at least 4 index ofrefraction changes greater than 0.2 in the vertical direction. Greaterthan 4 index of refraction changes have been shown to provide enoughdiffusion for most LC devices. 30 or more index of refractiondifferences in the vertical direction, while providing excellentdiffusion, significantly reduces the amount of transmitted light,significantly reducing the brightness of the LC device.

[0041] Since the thermoplastic light diffuser of the invention typicallyis used in combination with other optical web materials, a lightdiffuser with an elastic modulus greater than 500 MPa is preferred. Anelastic modulus greater than 500 MPa allows for the light diffuser to belaminated with a pressure sensitive adhesive for combination with otheroptical webs materials. Further, because the light diffuser ismechanically tough, the light diffuser is better able to with stand therigors of the assembly process compared to prior art cast diffusionfilms which are delicate and difficult to assemble. A light diffuserwith an impact resistance greater than 0.6 GPa is preferred. An impactresistance greater than 0.6 GPa allows the light diffuser to resistscratching and mechanical deformation that can cause unwanted unevendiffusion of the light causing “hot” spots in an LC device.

[0042] The thickness of the light diffuser preferably is less than 250micrometers. Current design trends for LC devices are toward lighter andthinner devices. By reducing the thickness of the light diffuser to lessthan 250 micrometers, the LC devices can be made lighter and thinner.Further, by reducing the thickness of the light diffuser, brightness ofthe LC device can be improved. The more preferred thickness of the lightdiffuser is between 12.5 and 50 micrometers which further allows thelight diffuser to be convienently combined with a other opticalmaterials in an LC device such as brightness enhancement films. Further,by reducing the thickness of the light diffuser, the materials contentof the diffuser is reduced.

[0043] The thickness uniformity of the light diffuser across thediffuser is preferably less than 0.10 micrometers. Thickness uniformityis defined as the diffuser thickness difference between the maximumdiffuser thickness and the minimum diffuser thickness. By orienting thelight diffuser of the invention, the thickness uniformity of thediffuser is less than 0.10 micrometers, allowing for a more uniformdiffusion of light across the LC device compared to cast coateddiffuser. As the LC market moves to larger sizes (40 cm diagonal orgreater), the uniformity of the light diffusion becomes an importantimage quality parameter. By providing a voided light diffuser withthickness uniformity less than 0.10 micrometers across the diffusionweb, the quality of image is maintained.

[0044] For light diffuser of the invention, micro-voided compositebiaxially oriented polyolefin sheets are preferred and are manufacturedby coextrusion of the core and surface layer(s), followed by biaxialorientation, whereby voids are formed around void-initiating materialcontained in the core layers. For the biaxially oriented layer, suitableclasses of thermoplastic polymers for the biaxially oriented sheet andthe core matrix-polymer of the preferred composite sheet comprisepolyolefins. Suitable polyolefins include polypropylene, polyethylene,polymethylpentene, polystyrene, polybutylene and mixtures thereof.Polyolefin copolymers, including copolymers of propylene and ethylenesuch as hexene, butene, and octene are also useful. Polyethylene ispreferred, as it is low in cost and has desirable strength properties.Such composite sheets are disclosed in, for example, U.S. Pat. Nos.4,377,616; 4,758,462 and 4,632,869, the disclosure of which isincorporated for reference. The light diffuser film comprises a polymersheet with at least one voided polymer layer and could contain nonvoidedpolyester polymer layer(s). It should comprise a void space betweenabout 2 and 60% by volume of said voided layer of said polymer sheet.Such a void concentration is desirable to optimize the transmission andreflective properties while providing adequate diffusing power to hideback lights and filaments. The thickness of the micro void-containingoriented film of the present invention is preferably about 1 micrometerto 400 micrometer, more preferably 5 micrometer to 200 micrometer. Apolymer sheet having a percent transmission greater than 65% at 500 nm.

[0045] The thermoplastic diffuser of the invention is preferablyprovided with a one or more nonvoided skin layers adjacent to themicrovoided layers. The nonvoided skin layers of the composite sheet canbe made of the same polymeric materials as listed above for the corematrixes. The composite sheet can be made with skin(s) of the samepolymeric material as the core matrix, or it can be made with skin(s) ofdifferent polymeric composition than the core matrix. For compatibility,an auxiliary layer can be used to promote adhesion of the skin layer tothe core layers. Any suitable polyester sheet may be utilized for themember provided that it is oriented. The orientation provides addedstrength to the multi-layer structure that provides enhanced handlingproperties when displays are assembled. Microvoided oriented sheets arepreferred because the voids provide opacity without the use of TiO₂.Microvoided layers are conveniently manufactured by coextrusion of thecore and thin layers, followed by biaxial orientation, whereby voids areformed around void-initiating material contained in the thin layers.

[0046] Polyester microvoided light diffusers are also preferred asoriented polyester has excellent strength, impact resistance andchemical resistance. The polyester utilized in the invention should havea glass transition temperature between about 50.degree. C. and about150.degree. C., preferably about 60-100.degree. C., should beorientable, and have an intrinsic viscosity of at least 0.50, preferably0.6 to 0.9. Suitable polyesters include those produced from aromatic,aliphatic, or cyclo-aliphatic dicarboxylic acids of 4-20 carbon atomsand aliphatic or alicyclic glycols having from 2-24 carbon atoms.Examples of suitable dicarboxylic acids include terephthalic,isophthalic, phthalic, naphthalene dicarboxylic acid, succinic,glutaric, adipic, azelaic, sebacic, fumaric, maleic, itaconic,1,4-cyclohexanedicarboxylic, sodiosulfoiso-phthalic, and mixturesthereof. Examples of suitable glycols include ethylene glycol, propyleneglycol, butanediol, pentanediol, hexanediol, 1,4-cyclohexane-dimethanol,diethylene glycol, other polyethylene glycols and mixtures thereof. Suchpolyesters are well known in the art and may be produced by well-knowntechniques, e.g., those described in U.S. Pat. Nos. 2,465,319 and2,901,466. Preferred continuous matrix polymers are those having repeatunits from terephthalic acid or naphthalene dicarboxylic acid and atleast one glycol selected from ethylene glycol, 1,4-butanediol, and1,4-cyclohexanedimethanol. Poly(ethylene terephthalate), which may bemodified by small amounts of other monomers, is especially preferred.Polypropylene is also useful. Other suitable polyesters include liquidcrystal copolyesters formed by the inclusion of a suitable amount of aco-acid component such as stilbene dicarboxylic acid. Examples of suchliquid crystal copolyesters are those disclosed in U.S. Pat. Nos.4,420,607; 4,459,402; and 4,468,510.

[0047] The coextrusion, quenching, orienting, and heat setting ofpolyester diffuser sheets may be effected by any process which is knownin the art for producing oriented sheet, such as by a flat sheet processor a bubble or tubular process. The flat sheet process involvesextruding the blend through a slit die and rapidly quenching theextruded web upon a chilled casting drum so that the core matrix polymercomponent of the sheet and the skin components(s) are quenched belowtheir glass solidification temperature. The quenched sheet is thenbiaxially oriented by stretching in mutually perpendicular directions ata temperature above the glass transition temperature, below the meltingtemperature of the matrix polymers. The sheet may be stretched in onedirection and then in a second direction or may be simultaneouslystretched in both directions. After the sheet has been stretched, it isheat set by heating to a temperature sufficient to crystallize or annealthe polymers while restraining to some degree the sheet againstretraction in both directions of stretching.

[0048] Additional layers preferably are added to the micro-voidedpolyester diffusion sheet which may achieve a different effect. Suchlayers might contain tints, antistatic materials, or differentvoid-making materials to produce sheets of unique properties. Biaxiallyoriented sheets could be formed with surface layers that would providean improved adhesion. The biaxially oriented extrusion could be carriedout with as many as 10 layers if desired to achieve some particulardesired property.

[0049] Addenda are preferably added to a polyester skin layer to changethe color of the film. Colored pigments that can resist extrusiontemperatures greater than 320. degree. C. are preferred as temperaturesgreater than 320. degree. C. are necessary for coextrusion of the skinlayer.

[0050] An addenda of this invention that could be added is an opticalbrightener. An optical brightener is substantially colorless,fluorescent, organic compound that absorbs ultraviolet light and emitsit as visible blue light. Examples include but are not limited toderivatives of 4,4′-diaminostilbene-2,2′-disulfonic acid, coumarinderivatives such as 4-methyl-7-diethylaminocoumarin, 1-4-Bis(O-Cyanostyryl) Benzol and 2-Amino-4-Methyl Phenol. An unexpecteddesirable feature of this efficient use of optical brightener. Becausethe ultraviolet source for a transmission display material is on theopposite side of the image, the ultraviolet light intensity is notreduced by ultraviolet filters common to imaging layers. The result isless optical brightener is required to achieve the desired backgroundcolor.

[0051] The polyester diffuser sheets may be coated or treated after thecoextrusion and orienting process or between casting and fullorientation with any number of coatings which may be used to improve theproperties of the sheets including printability, to provide a vaporbarrier, to make them heat sealable, or to improve adhesion. Examples ofthis would be acrylic coatings for printability, coating polyvinylidenechloride for heat seal properties. Further examples include flame,plasma or corona discharge treatment to improve printability oradhesion. By having at least one nonvoided skin on the micro-voidedcore, the tensile strength of the sheet is increased and makes it moremanufacturable. It allows the sheets to be made at wider widths andhigher draw ratios than when sheets are made with all layers voided. Thenon-voided layer(s) can be peeled off after manufacture of the film.Coextruding the layers further simplifies the manufacturing process.

[0052] The oriented thermoplastic diffuser sheets of the presentinvention may be used in combination with one or more layers selectedfrom an optical compensation film, a polarizing film and a substrateconstitution a liquid crystal layer. The oriented film of the presentinvention is preferably used by a combination of orientedfilm/polarizing film/optical compensation film in the order. In the caseof using the above films in combination in a liquid crystal displaydevice, the films are preferably bonded with each other e.g. through atacky adhesive for minimizing the reflection loss, etc. The tackyadhesive is preferably those having a refractive index close to that ofthe oriented film to suppress the interfacial reflection loss of light.

[0053] The oriented thermoplastic diffusion sheet of the presentinvention may be used in combination with a film or sheet made of atransparent polymer. Examples of such polymer are polyesters such aspolycarbonate, polyethylene terephthalate, polybutylene terephthalateand polyethylene naphthalate, acrylic polymers such as polymethylmethacrylate, and polyethylene, polypropylene, polystyrene, polyvinylchloride, polyether sulfone, polysulfone, polyarylate and triacetylcellulose.

[0054] The oriented thermoplastic diffuser sheet of the presentinvention may be incorporated with e.g. an additive or a lubricant suchas silica for improving the drawability and the surface-slipperiness ofthe film within a range not to deteriorate the optical characteristicsto vary the light-scattering property with an incident angle. Examplesof such additive are organic solvents such as xylene, alcohols orketones, fine particles of an acrylic resin, silicone resin or A metaloxide or a filler.

[0055] The micro void-containing oriented film of the present inventionusually has optical anisotropy. A biaxially drawn film of athermoplastic polymer is generally an optically anisotropic materialexhibiting optical anisotropy having an optic axis in the drawingdirection. The optical anisotropy is expressed by the product of thefilm thickness d and the birefringence Δn which is a difference betweenthe refractive index in the slow optic axis direction and the refractiveindex in the fast optic axis direction in the plane of the film, i.e.Δn.multidot.d (retardation). The orientation direction coincides withthe drawing axis in the film of the present invention. The drawing axisis the direction of the slow optic axis in the case of a thermoplasticpolymer having a positive intrinsic birefringence and is the directionof the fast optic axis for a thermoplastic polymer having a negativeintrinsic birefringence. There is no definite requirement for thenecessary level of the value of Δn.multidot.d since the level dependsupon the application of the film, however, it is preferably 50 nm ormore.

[0056] The microvoid-containing oriented film of the present inventionhas a function to diffuse the light. A periodically varying refractiveindex distribution formed by these numerous microvoids and micro-lensformed by the microvoided layers forms a structure like a diffractiongrating to contribute to the optical property to scatter the light. Thevoided thermoplastic diffuser sheet provides excellent scattering oflight while having a high % light transmission. “Void” is used herein tomean devoid of added solid and liquid matter, although it is likely the“voids” contain gas. The void-initiating particles which remain in thefinished diffuser sheet core should be from 0.1 to 10 micrometers indiameter, preferably round in shape, to produce voids of the desiredshape and size. Voids resulting from the use of initiating particles ofthis size are termed “microvoids” herein. The voids exhibit a dimensionof 10 micrometeres or less in the unoriented thickness or Z direction ofthe layer. The size of the void is also dependent on the degree oforientation in the machine and transverse directions. Ideally, the voidwould assume a shape which is defined by two opposed and edge contactingconcave disks. In other words, the voids tend to have a substantiallycircular cross section in the plane perpendicular to the direction ofthe light energy (also termed the vertical direction herein). The voidsare oriented so that the two major dimensions (major axis and minoraxis) are aligned with the machine and transverse directions of thesheet. The Z-direction axis is a minor dimension and is roughly the sizeof the cross diameter of the voiding particle. The voids generally tendto be closed cells, and thus there is virtually no path open from oneside of the voided-core to the other side through which gas or liquidcan traverse.

[0057] Microvoids formed from organic spheres are preferred because theyare low in light scattering, have been shown to form substantiallycircular voids and are easily dispersed in polyester. Further, the sizeand the shape of the voided diffuser layer can be changed by properselection of organic sphere size and amount. Microvoids that aresubstantially free of scattering inorganic particles is also preferred.Prior art voided polymer layers that use inorganic particles such asclay, TiO₂, and silica have been shown to unacceptably scatter visiblelight energy. Scattering light energy from the back light source reducesthe efficiency of the display unit by scattering light energy away fromthe LC and back toward the light source. It has been shown thatinorganic microvoiding particles can cause as much as 8% loss intransmitted light energy.

[0058] A microvoid is a void in the polymer layer of the diffuser thathas a volume less than 100 micrometers. Microvoids larger than 100micrometers are capable of diffusing visible light, however, because thevoid size is large, uneven diffusion of the light occurs resulting inuneven lighting of display devices. A thermoplastic microvoid volumebetween 8 and 42 cubic micrometers is preferred. A microvoided volumeless than 6 cubic micrometers is difficult to obtain as the voidingagent required for 6 cubic micrometers is to small to void with typical3×3 orientation of polyester. A microvoid volume greater than 50 cubicmicrometers, while providing diffusion, creates a thick diffusion layerrequiring extra material and cost. The most preferred void volume forthe thermoplastic diffuser is between 10 and 20 cubic micrometers.Between 10 and 20 cubic micrometers has been shown to provide excellentdiffusion and transmission properties.

[0059] The organic void-initiating material may be selected from avariety of materials, and should be present in an amount of about 5 to50% by weight based on the weight of the core matrix polymer.Preferably, the void-initiating material comprises a polymeric material.When a polymeric material is used, it may be a polymer that can bemelt-mixed with the polymer from which the core matrix is made and beable to form dispersed spherical particles as the suspension is cooleddown. Examples of this would include nylon dispersed in polypropylene,polybutylene terephthalate in polypropylene, or polypropylene dispersedin polyethylene terephthalate. If the polymer is preshaped and blendedinto the matrix polymer, the important characteristic is the size andshape of the particles. Spheres are preferred and they can be hollow orsolid. These spheres may be made from cross-linked polymers which aremembers selected from the group consisting of an alkenyl aromaticcompound having the general formula Ar—C(R)═CH₂, wherein Ar representsan aromatic hydrocarbon radical, or an aromatic halohydrocarbon radicalof the benzene series and R is hydrogen or the methyl radical;acrylate-type monomers include monomers of the formula CH₂═C(R′)C(O)(OR)wherein R is selected from the group consisting of hydrogen and an alkylradical containing from about 1 to 12 carbon atoms and R′ is selectedfrom the group consisting of hydrogen and methyl; copolymers of vinylchloride and vinylidene chloride, acrylonitrile and vinyl chloride,vinyl bromide, vinyl esters having formula CH₂═CH(O)COR, wherein R is analkyl radical containing from 2 to 18 carbon atoms; acrylic acid,methacrylic acid, itaconic acid, citraconic acid, maleic acid, fumaricacid, oleic acid, vinylbenzoic acid; the synthetic polyester resinswhich are prepared by reacting terephthalic acid and dialkylterephthalics or ester-forming derivatives thereof, with a glycol of theseries HO(CH.sub.2).sub.n OH wherein n is a whole number within therange of 2-10 and having reactive olefinic linkages within the polymermolecule, the above described polyesters which include copolymerizedtherein up to 20 percent by weight of a second acid or ester thereofhaving reactive olefinic unsaturation and mixtures thereof, and across-linking agent selected from the group consisting ofdivinylbenzene, diethylene glycol dimethacrylate, diallyl fumarate,diallyl phthalate, and mixtures thereof

[0060] Preferred crosslinked polymer beads have a mean particle sizeless than 2.0 micrometers, more preferably between 0.3 and 1.7micrometers. Crosslinked polymer beads smaller than 0.3 micrometers areprohibitivly expensive. Crosslinked polymer beads larger than 1.7micrometers make voids that large and do not scatter light efficiently.

[0061] Suitable cross-linked polymers for the microbeads used in voidformation during sheet formation are polymerizable organic materialswhich are members selected from the group consisting of an alkenylaromatic compound having the general formula

[0062] wherein Ar represents an aromatic hydrocarbon radical, or anaromatic halohydrocarbon radical of the benzene series and R is hydrogenor the methyl radical; acrylate-type monomers including monomers of theformula

[0063] wherein R is selected from the group consisting of hydrogen andan alkyl radical containing from about 1 to 12 carbon atoms and R′ isselected from the group consisting of hydrogen and methyl; copolymers ofvinyl chloride and vinylidene chloride, acrylonitrile and vinylchloride, vinyl bromide, vinyl esters having the formula

[0064] wherein R is an alkyl radical containing from 2 to 18 carbonatoms; acrylic acid, methacrylic acid, itaconic acid, citraconic acid,maleic acid, fumaric acid, oleic acid, vinylbenzoic acid; the syntheticpolyester resins which are prepared by reacting terephthalic acid anddialkyl terephthalics or ester-forming derivatives thereof, with aglycol of the series HO(CH₂)_(n)OH, wherein n is a whole number withinthe range of 2-10 and having reactive olefinic linkages within thepolymer molecule, the hereinabove described polyesters which includecopolymerized therein up to 20 percent by weight of a second acid orester thereof having reactive olefinic unsaturation and mixturesthereof, and a cross-linking agent selected from the group consisting ofdivinyl-benzene, diethylene glycol dimethacrylate, oiallyl fumarate,diallyl phthalate, and mixtures thereof.

[0065] Examples of typical monomers for making the cross-linked polymerinclude styrene, butyl acrylate, acrylamide, acrylonitrile, methylmethacrylate, ethylene glycol dimethacrylate, vinyl pyridine, vinylacetate, methyl acrylate, vinylbenzyl chloride, vinylidene chloride,acrylic acid, divinylbenzene, arrylamidomethyl-propane sulfonic acid,vinyl toluene, etc. Preferably, the cross-linked polymer is polystyreneor poly(methyl methacrylate). Most preferably, it is polystyrene and thecross-linking agent is divinylbenzene.

[0066] Processes well known in the art yield nonuniformly sizedparticles, characterized by broad particle size distributions. Theresulting beads can be classified by screening to produce beads spanningthe range of the original distribution of sizes. Other processes such assuspension polymerization and limited coalescence directly yield veryuniformly sized particles. U.S. Pat. No. 6,074,788, the disclosure ofwhich is incorporated for reference. It is preferred to use the “limitedcoalescance” technique for producing the coated, cross-linked polymermicrobeads. This process is described in detail in U.S. Pat. No.3,615,972. Preparation of the coated microbeads for use in the presentinvention does not utilize a blowing agent as described in this patent,however. Suitable slip agents or lubricants include colloidal silica,colloidal alumina, and metal oxides such as tin oxide and aluminumoxide. The preferred slip agents are colloidal silica and alumina, mostpreferably, silica. The cross-linked polymer having a coating of slipagent may be prepared by procedures well known in the art. For example,conventional suspension polymerization processes wherein the slip agentis added to the suspension is preferred. As the slip agent, colloidalsilica is preferred.

[0067] The microbeads of cross-linked polymer range in size from 0.1-50.mu.m, and are present in an amount of 5-50% by weight based on theweight of the polyester. Microbeads of polystyrene should have a Tg ofat least 20° C. higher than the Tg of the continuous matrix polymer andare hard compared to the continuous matrix polymer.

[0068] Elasticity and resiliency of the microbeads generally result inincreased voiding, and it is preferred to have the Tg of the microbeadsas high above that of the matrix polymer as possible to avoiddeformation during orientation. It is not believed that there is apractical advantage to cross-linking above the point of resiliency andelasticity of the microbeads. The microbeads of cross-linked polymer areat least partially bordered by voids. The void space in the supportsshould occupy 2-60%, preferably 30-50%, by volume of the film support.Depending on the manner in which the supports are made, the voids maycompletely encircle the microbeads, e.g., a void may be in the shape ofa doughnut (or flattened doughnut) encircling a micro-bead, or the voidsmay only partially border the microbeads, e.g., a pair of voids mayborder a microbead on opposite sides.

[0069] During stretching the voids assume characteristic shapes from thebalanced biaxial orientation of films to the uniaxial orientation ofmicrovoided films. Balanced microvoids are largely circular in the planeof orientation. The size of the microvoids and the ultimate physicalproperties depend upon the degree and balance of the orientation,temperature and rate of stretching, crystallization kinetics, the sizedistribution of the microbeads, and the like. The film supportsaccording to this invention are prepared by: (a) forming a mixture ofmolten continuous matrixpolymer and cross-linked polymer wherein thecross-linked polymer is a multiplicity of microbeads uniformly dispersedthroughout the matrix polymer, the matrix polymer being as describedhereinbefore, the cross-linked polymer microbeads being as describedhereinbefore, (b) forming a film support from the mixture by extrusionor casting, (c) orienting the article by stretching to form microbeadsof cross-linked polymer uniformly distributed throughout the article andvoids at least partially bordering the microbeads on sides thereof inthe direction, or directions of orientation.

[0070] Methods of bilaterally orienting sheet or film material are wellknown in the art. Basically, such methods comprise stretching the sheetor film at least in the machine or longitudinal direction after it iscast or extruded an amount of about 1.5-10 times its original dimension.Such sheet or film may also be stretched in the transverse orcross-machine direction by apparatus and methods well known in the art,in amounts of generally 1.5-10 (usually 3-4 for polyesters and 6-10 forpolypropylene) times the original dimension. Such apparatus and methodsare well known in the art and are described in such U.S. Pat. No.3,903,234.

[0071] The voids, or void spaces, referred to herein surrounding themicrobeads are formed as the continuous matrix polymer is stretched at atemperature above the Tg of the matrix polymer. The microbeads ofcross-linked polymer are relatively hard compared to the continuousmatrix polymer. Also, due to the incompatibility and immiscibilitybetween the microbead and the matrix polymer, the continuous matrixpolymer slides over the microbeads as it is stretched, causing voids tobe formed at the sides in the direction or directions of stretch, whichvoids elongate as the matrix polymer continues to be stretched. Thus,the final size and shape of the voids depends on the direction(s) andamount of stretching. If stretching is only in one direction, microvoidswill form at the sides of the microbeads in the direction of stretching.If stretching is in two directions (bidirectional stretching), in effectsuch stretching has vector components extending radially from any givenposition to result in a doughnut-shaped void surrounding each microbead.

[0072] The preferred preform stretching operation simultaneously opensthe microvoids and orients the matrix material. The final productproperties depend on and can be controlled by stretchingtime-temperature relationships and on the type and degree of stretch.For maximum opacity and texture, the stretching is done just above theglass transition temperature of the matrix polymer. When stretching isdone in the neighborhood of the higher glass transition temperature,both phases may stretch together and opacity decreases. In the formercase, the materials are pulled apart, a mechanical anticompatibilizationprocess. In general, void formation occurs independent of, and does notrequire, crystalline orientation of the matrix polymer. Opaque,microvoided films have been made in accordance with the methods of thisinvention using completely amorphous, noncrystallizing copolyesters asthe matrix phase. Crystallizable/orientable (strain hardening) matrixmaterials are preferred for some properties like tensile strength andgas transmission barrier. On the other hand, amorphous matrix materialshave special utility in other areas like tear resistance and heatsealability. The specific matrix composition can be tailored to meetmany product needs. The complete range from crystalline to amorphousmatrix polymer is part of the invention.

[0073] The invention may be used in conjunction with any liquid crystaldisplay devices, typical arrangements of which are described in thefollowing. Liquid crystals (LC) are widely used for electronic displays.In these display systems, an LC layer is situated between a polarizerlayer and an analyzer layer and has a director exhibiting an azimuthaltwist through the layer with respect to the normal axis. The analyzer isoriented such that its absorbing axis is perpendicular to that of thepolarizer. Incident light polarized by the polarizer passes through aliquid crystal cell is affected by the molecular orientation in theliquid crystal, which can be altered by the application of a voltageacross the cell. By employing this principle, the transmission of lightfrom an external source, including ambient light, can be controlled. Theenergy required to achieve this control is generally much less than thatrequired for the luminescent materials used in other display types suchas cathode ray tubes. Accordingly, LC technology is used for a number ofapplications, including but not limited to digital watches, calculators,portable computers, electronic games for which light weight, low powerconsumption and long operating life are important features.

[0074] Active-matrix liquid crystal displays (LCDs) use thin filmtransistors (TFTs) as a switching device for driving each liquid crystalpixel. These LCDs can display higher-definition images without crosstalk because the individual liquid crystal pixels can be selectivelydriven. Optical mode interference (OMI) displays are liquid crystaldisplays, which are “normally white,” that is, light is transmittedthrough the display layers in the off state. Operational mode of LCDusing the twisted nematic liquid crystal is roughly divided into abirefringence mode and an optical rotatory mode. “Film-compensatedsuper-twisted nematic” (FSTN) LCDs are normally black, that is, lighttransmission is inhibited in the off state when no voltage is applied.OMI displays reportedly have faster response times and a broaderoperational temperature range.

[0075] Ordinary light from an incandescent bulb or from the sun israndomly polarized, that is, it includes waves that are oriented in allpossible directions. A polarizer is a dichroic material that functionsto convert a randomly polarized (“unpolarized”) beam of light into apolarized one by selective removal of one of the two perpendicularplane-polarized components from the incident light beam. Linearpolarizers are a key component of liquid-crystal display (LCD) devices.

[0076] There are several types of high dichroic ratio polarizerspossessing sufficient optical performance for use in LCD devices. Thesepolarizers are made of thin sheets of materials which transmit onepolarization component and absorb the other mutually orthogonalcomponent (this effect is known as dichroism). The most commonly usedplastic sheet polarizers are composed of a thin, uniaxially-stretchedpolyvinyl alcohol (PVA) film which aligns the PVA polymer chains in amore-or-less parallel fashion. The aligned PVA is then doped with iodinemolecules or a combination of colored dichroic dyes (see, for example,EP 0 182 632 A2, Sumitomo Chemical Company, Limited) which adsorb to andbecome uniaxially oriented by the PVA to produce a highly anisotropicmatrix with a neutral gray coloration. To mechanically support thefragile PVA film it is then laminated on both sides with stiff layers oftriacetyl cellulose (TAC), or similar support.

[0077] Contrast, color reproduction, and stable gray scale intensitiesare important quality attributes for electronic displays, which employliquid crystal technology. The primary factor limiting the contrast of aliquid crystal display is the propensity for light to “leak” throughliquid crystal elements or cell, which are in the dark or “black” pixelstate. Furthermore, the leakage and hence contrast of a liquid crystaldisplay are also dependent on the angle from which the display screen isviewed. Typically the optimum contrast is observed only within a narrowviewing angle centered about the normal incidence to the display andfalls off rapidly as the viewing angle is increased. In color displays,the leakage problem not only degrades the contrast but also causes coloror hue shifts with an associated degradation of color reproduction. Inaddition to black-state light leakage, the narrow viewing angle problemin typical twisted nematic liquid crystal displays is exacerbated by ashift in the brightness-voltage curve as a function of viewing anglebecause of the optical anisotropy of the liquid crystal material.

[0078] The micro-voided homogenizing polymer film of the presentinvention can even out the luminance when the film is used as alight-scattering film in a backlight system. Back-lit LCD displayscreens, such as are utilized in portable computers, may have arelatively localized light source (ex. fluorescent light) or an array ofrelatively localized light sources disposed relatively close to the LCDscreen, so that individual “hot spots” corresponding to the lightsources may be detectable. The micro-voided polymer film serves to evenout the illumination across the display. The liquid crystal displaydevice includes display devices having a combination of a driving methodselected from e.g. active matrix driving and simple matrix drive and aliquid crystal mode selected from e.g. twist nematic, supertwistnematic, ferroelectric liquid crystal and antiferroelectric liquidcrystal mode, however, the invention is not restricted by the abovecombinations. In a liquid crystal display device, the oriented film ofthe present invention is necessary to be positioned in front of thebacklight. The micro-voided polymer film of the present invention caneven the lightness of a liquid crystal display device across the displaybecause the film has excellent light-scattering properties to expand thelight to give excellent visibility in all directions. Although the aboveeffect can be achieved even by the single use of such oriented film,plural number of films may be used in combination. The homogenizingmicro-voided polymer film may be placed in front of the LCD material ina transmission mode to disburse the light and make it much morehomogenous. The present invention has a significant use as a lightsource destructuring device. In many applications, it is desirable toeliminate from the output of the light source itself the structure ofthe filament which can be problematic in certain applications becauselight distributed across the sample will vary and this is undesirable.Also, variances in the orientation of a light source filament or arcafter a light source is replaced can generate erroneous and misleadingreadings. A homogenizing micro-voided film of the present inventionplaced between the light source and the detector can eliminate from theoutput of the light source any trace of the filament structure andtherefore causes a homogenized output which is identical from lightsource to light source.

[0079] The micro-voided polymer films may be used to control lightingfor stages by providing pleasing homogenized light that is directedwhere desired. In stage and television productions, a wide variety ofstage lights must be used to achieve all the different effects necessaryfor proper lighting. This requires that many different lamps be usedwhich is inconvenient and expensive. The films of the present inventionplaced over a lamp can give almost unlimited flexibility dispersinglight where it is needed. As a consequence, almost any object, moving ornot, and of any shape, can be correctly illuminated.

[0080] The reflection film formed by applying a reflection layercomposed of a metallic film, etc., to the oriented film of the presentinvention can be used e.g. as a retroreflective member for a trafficsign. It can be used in a state applied to a car, a bicycle, person,etc.

[0081] The micro-voided films of the present invention may also be usedin the area of law enforcement and security systems to homogenize theoutput from laser diodes (LDs) or light emitting diodes (LEDs) over theentire secured area to provide higher contrasts to infrared (IR)detectors. The films of the present invention may also be used to removestructure from devices using LED or LD sources such as in bank notereaders or skin treatment devices. This leads to greater accuracy.

[0082] Fiber-optic light assemblies mounted on a surgeon's headpiece cancast distracting intensity variations on the surgical field if one ofthe fiber-optic elements breaks during surgery. A micro-voided film ofthe present invention placed at the ends of the fiber bundle homogenizeslight coming from the remaining fibers and eliminates any trace of thebroken fiber from the light cast on the patient. A standard ground glassdiffuser would not be as effective in this use due to significantback-scatter causing loss of throughput.

[0083] The micro-voided polymer films of the present invention can alsobe used to homogeneously illuminate a sample under a microscope bydestructuring the filament or arc of the source, yielding ahomogeneously illuminated field of view. The films may also be used tohomogenize the various modes that propagate through a fiber, forexample, the light output from a helical-mode fiber.

[0084] The voided polymer films of the present invention also havesignificant architectural uses such as providing appropriate light forwork and living spaces. In typical commercial applications, inexpensivemicro-voided plastic sheets are used to help diffuse light over theroom. A homogenizer of the present invention which replaces one of theseconventional diffusers provides a more uniform light output so thatlight is diffused to all angles across the room evenly and with no hotspots.

[0085] The voided polymer films of the present invention may also beused to diffuse light illuminating artwork. The voided polymer filmprovides a suitable appropriately sized and directed aperture fordepicting the artwork in a most desirable fashion.

[0086] Further, the oriented film of the present invention can be usedwidely as a part for optical equipment such as a displaying device. Forexample, it can be used as a light-reflection plate laminated with areflection film such as a metal film in a reflective liquid crystaldisplay device or a front scattering film directing the film to thefront-side (observer's side) in the case of placing the metallic film tothe back side of the device (opposite to the observer), in addition tothe aforementioned light-scattering plate of a backlight system of aliquid crystal display device. The micro-voided oriented film of thepresent invention can be used as an electrode by laminating atransparent conductive layer composed of indium oxide represented by ITOfilm. If the material is to be used to form a reflective screen, e.g.front projection screen, a light-reflective layer is applied to thevoided polymer surface.

[0087] In another embodiment of the invention, the thermoplasticdiffusion layer of the invention is preferably formed from a polymerfoam process. The polymer foam process allows for the formation of airvoids in a polymer matrix providing a index of refraction differencebetween the air voids and the polymer matrix of greater than 0.2. Sincethe polymer air forming process creates air voids without the use of avoiding agent, no light energy scattering has been observed. The foamingof these polymers may be carried out through several mechanical,chemical, or physical means. Mechanical methods include whipping a gasinto a polymer melt, solution, or suspension, which then hardens eitherby catalytic action or heat or both, thus entrapping the gas bubbles inthe matrix. Chemical methods include such techniques as the thermaldecomposition of chemical blowing agents generating gases such asnitrogen or carbon dioxide by the application of heat or throughexothermic heat of reaction during polymerization. Physical methodsinclude such techniques as the expansion of a gas dissolved in a polymermass upon reduction of system pressure; the volatilization oflow-boiling liquids such as fluorocarbons or methylene chloride, or theincorporation of hollow microspheres in a polymer matrix. The choice offoaming technique is dictated by desired foam density reduction, desiredproperties, and manufacturing process.

[0088] In a preferred embodiment of this invention polyolefins such aspolyethylene and polypropylene, their blends and their copolymers areused as the matrix polymer in the foam core along with a chemicalblowing agent such as sodium bicarbonate and its mixture with citricacid, organic acid salts, azodicarbonamide, azobisformamide,azobisisobutyrolnitrile, diazoaminobenzene, 4,4′-oxybis(benzene sulfonylhydrazide) (OBSH), N,N′-dinitrosopentamethyltetramine (DNPA), sodiumborohydride, and other blowing agent agents well known in the art. Thepreferred chemical blowing agents would be sodium bicarbonate/citricacid mixtures, azodicarbonamide, though others can also be used. Ifnecessary, these foaming agents may be used together with an auxiliaryfoaming agent, nucleating agent, and a cross-linking agent.

[0089] Diffusion film samples were measured with the Hitachi U4001UV/Vis/NIR spectrophotometer equipped with an integrating sphere. Thetotal transmittance spectra were measured by placing the samples at thebeam port with the voided side towards the integrating sphere. Acalibrated 99% diffusely reflecting standard (NIST-traceable) was placedat the normal sample port. The diffuse transmittance spectra weremeasured in like manner, but with the 99% tile removed. The diffusereflectance spectra were measured by placing the samples at the sampleport with the coated side towards the integrating sphere. In order toexclude reflection from a sample backing, nothing was placed behind thesample. All spectra were acquired between 350 and 800 nm. As the diffusereflectance results are quoted with respect to the 99% tile, the valuesare not absolute, but would need to be corrected by the calibrationreport of the 99% tile.

[0090] Percentage total transmitted light refers to percent of lightthat is transmitted though the sample at all angles. Diffusetransmittance is defined as the percent of light passing though thesample excluding a 2 degree angle from the incident light angle. Thediffuse light transfer efficiency is the percent of light that is passedthrough the sample by diffuse transmittance. Diffuse reflectance isdefined as the percent of light reflected by the sample. The percentagesquoted in the examples were measured at 500 nm. These values may not addup to 100% due to absorbencies of the sample or slight variations in thesample measured.

[0091] Embodiments of the invention may provide not only improved lightdiffusion and transmission but also a diffusion film of reducedthickness, that eliminates the need for an air gap, and that has reducedlight scattering tendencies.

[0092] The invention has been described in detail with particularreference to certain preferred embodiments thereof, but it will beunderstood that variations and modifications can be effected within thespirit and scope of the invention.

EXAMPLES

[0093] In this series of examples, commercially available polyesterpolymer was melt extruded with organic voiding beads. This the examplesbelow stretch extent, void size and thickness were varied to produce aseries of LC (liquid crystal) diffuser sheets including the ‘stacked’microvoided sheet of the invention. The examples below will show thatmicrovoided polyester polymer diffuser sheets provide excellent lightdiffusion and high light transmission, both of which are required forthe demanding LC component market and that ‘stacked’ microvoided sheetsare advantaged to a monolayer diffuser film.

Example 1

[0094] A transparent amorphous film composed of three layers having anoverall width of 16 cm was manufactured by a co-extrusion process. Oneof the outer layers, hereafter referred to as layer (A), was composed ofpoly(ethylene terephthalate) (“PET”, commercially available from EastmanChemical Company as Eastapak #7352). The intrinsic viscosity (I.V.) ofthe PET 7352 resin was 0.74. This layer was approximately 245 μm inthickness. The center layer, hereafter referred to as layer (B), wascomposed of PET (commercially available from Eastman Chemical Company asEastapak #9921) impregnated with a particulate voiding agent. Theintrinsic viscosity (I.V.) of the PET 9921 resin was 0.80. This layerwas approximately 30 μm in thickness. The remaining outer layer,hereafter referred to as layer (C), was composed of PET 9921 andimpregnated with a particulate voiding agent of a different size thanthe voiding agent in layer (B). This layer was approximately 48 μm inthickness. All voided layers in each example were impregnated withapproximately the same concentration of void initiating beads and thushave approximately the same frequency of voids.

[0095] The particulate voiding agents were created as follows. A 27 mmtwin screw compounding extruder heated to 275° C. was used to mixpolystyrene beads cross-linked with divinylbenzene with PET 9921. Thebeads used for layer (B) had an average particle diameter of 2 μm. Thebeads used for layer (C) had an average particle diameter of 5 μm. Thebeads were added to attain a 20% by weight loading in the PET 9921matrix. The components were metered into the compounder and one pass wassufficient for dispersion of the beads into the polyester matrix. Thedifferent size beads were compounded in two separate batches. Thecompounded material was extruded through a strand die, cooled in a waterbath, and pelletized.

[0096] Prior to the film co-extrusion process, the PET 7352 resin andthe compounded pellets were dried separately in desiccated driers at150° C. for 12 hours. The cast sheet was co-extruded in an A/B/C layerstructure. A standard 3.18 cm diameter screw extruder was used toextrude the PET 7352 resin for layer (A). A standard 1.91 cm diameterscrew extruder was used to extrude the compounded pellets for layer (B).A standard 3.18 cm diameter screw extruder was used to extrude thecompounded pellets for layer (C). The 275° C. melt streams were fed intoa 7 inch multi-manifold die also heated at 275° C. As the extruded sheetemerged from the die, it was cast onto a quenching roll set at 60-70° C.

[0097] The amorphous cast sheet was cut into 13 cm×13 cm squares. Thesheet was then stretched simultaneously in the X and Y-directions usinga standard laboratory film stretching unit. The cast sheet was stretchedsymmetrically in the X and Y-directions to an extent of approximately 2times the original sheet dimensions. The sheet temperature duringstretching was 103° C. The processing conditions are shown in Table 1.

Example 2

[0098] A transparent amorphous film composed of three layers having anoverall width of 16 cm was manufactured by a co-extrusion process asdescribed in Example 1. Layer (A), composed of PET 7352, wasapproximately 245 μm in thickness. Layer (B), composed of PET 9921impregnated with cross-linked polystyrene as a particulate voiding agenthaving an average particle size of 2 μm, was approximately 30 μm inthickness. Layer (C), composed of PET 9921 impregnated with cross-linkedpolystyrene as a particulate voiding agent having an average particlesize of 5 μm, was approximately 48 μm in thickness. The polymerscomposing the layers were processed as described in Example 1.

[0099] The amorphous cast sheet was stretched symmetrically in a similarfashion as described in Example 1. The sheet was stretched symmetricallyin the X and Y-directions to an extent of approximately 2.5 times theoriginal sheet dimensions. The sheet temperature during stretching was103° C. The processing conditions are shown in Table 1.

Example 3

[0100] A transparent amorphous film composed of three layers having anoverall width of 16 cm was manufactured by a co-extrusion process asdescribed in Example 1. Layer (A), composed of PET 7352, wasapproximately 245 μm in thickness. Layer (B), composed of PET 9921impregnated with cross-linked polystyrene as a particulate voiding agenthaving an average particle size of 2 μm, was approximately 30 μm inthickness. Layer (C), composed of PET 9921 impregnated with cross-linkedpolystyrene as a particulate voiding agent having an average particlesize of 5 μm, was approximately 48 μm in thickness. The polymerscomposing the layers were processed as described in Example 1.

[0101] The amorphous cast sheet was stretched symmetrically in a similarfashion as described in Example 1. The sheet was stretched symmetricallyin the X and Y-directions to an extent of approximately 3 times theoriginal sheet dimensions. The sheet temperature during stretching was103° C. The processing conditions are shown in Table 1.

Example 4

[0102] A transparent amorphous film composed of three layers having anoverall width of 16 cm was manufactured by a co-extrusion process asdescribed in Example 1. Layer (A), composed of PET 7352, wasapproximately 245 μm in thickness. Layer (B), composed of PET 9921impregnated with cross-linked polystyrene as a particulate voiding agenthaving an average particle size of 2 μm, was approximately 30 μm inthickness. Layer (C), composed of PET 9921 impregnated with cross-linkedpolystyrene as a particulate voiding agent having an average particlesize of 5 μm, was approximately 48 μm in thickness. The polymerscomposing the layers were processed as described in Example 1.

[0103] The amorphous cast sheet was stretched symmetrically in a similarfashion as described in Example 1. The sheet was stretched symmetricallyin the X and Y-directions to an extent of approximately 3.5 times theoriginal sheet dimensions. The sheet temperature during stretching was103° C. The processing conditions are shown in Table 1.

Example 5

[0104] A transparent amorphous film composed of three layers having anoverall width of 16 cm was manufactured by a co-extrusion process asdescribed in Example 1. Layer (A), composed of PET 7352, wasapproximately 245 μm in thickness. Layer (B), composed of PET 9921impregnated with cross-linked polystyrene as a particulate voiding agenthaving an average particle size of 2 μm, was approximately 30 μm inthickness. Layer (C), composed of PET 9921 impregnated with cross-linkedpolystyrene as a particulate voiding agent having an average particlesize of 5 μm, was approximately 48 μm in thickness. The polymerscomposing the layers were processed as described in Example 1.

[0105] The amorphous cast sheet was stretched symmetrically in a similarfashion as described in Example 1. The sheet was stretched symmetricallyin the X and Y-directions to an extent of approximately 4 times theoriginal sheet dimensions. The sheet temperature during stretching was103° C. The processing conditions are shown in Table 1.

Example 6

[0106] A transparent amorphous film composed of two layers having anoverall width of 16 cm was manufactured by a co-extrusion process asdescribed in Example 1. However, layer (C) was omitted from theco-extrusion process. Layer (A), composed of PET 7352, was approximately245 μm in thickness. Layer (B), composed of PET 9921 impregnated withcross-linked polystyrene as a particulate voiding agent having anaverage particle size of 2 Am, was approximately 30 μm in thickness. Thepolymers composing the layers were processed as described in Example 1.

[0107] The amorphous cast sheet was stretched symmetrically in a similarfashion as described in Example 1. The sheet was stretched symmetricallyin the X and Y-directions to an extent of approximately 3 times theoriginal sheet dimensions. The sheet temperature during stretching was103° C. The processing conditions are shown in Table 1.

Example 7

[0108] A transparent amorphous film composed of two layers having anoverall width of 16 cm was manufactured by a co-extrusion process asdescribed in Example 1. However, layer (C) was omitted from theco-extrusion process. Layer (A), composed of PET 7352, was approximately245 μm in thickness. Layer (B), composed of PET 9921 impregnated withcross-linked polystyrene as a particulate voiding agent having anaverage particle size of 2 μm, was approximately 30 μm in thickness. Thepolymers composing the layers were processed as described in Example 1.

[0109] The amorphous cast sheet was stretched symmetrically in a similarfashion as described in Example 1. The sheet was stretched symmetricallyin the X and Y-directions to an extent of approximately 3.5 times theoriginal sheet dimensions. The sheet temperature during stretching was103° C. The processing conditions are shown in Table 1.

Example 8

[0110] A transparent amorphous film composed of two layers having anoverall width of 16 cm was manufactured by a co-extrusion process asdescribed in Example 1. However, layer (C) was omitted from theco-extrusion process. Layer (A), composed of PET 7352, was approximately245 μm in thickness. Layer (B), composed of PET 9921 impregnated withcross-linked polystyrene as a particulate voiding agent having anaverage particle size of 2 μm, was approximately 30 μm in thickness. Thepolymers composing the layers were processed as described in Example 1.

[0111] The amorphous cast sheet was stretched symmetrically in a similarfashion as described in Example 1. The sheet was stretched symmetricallyin the X and Y-directions to an extent of approximately 4 times theoriginal sheet dimensions. The sheet temperature during stretching was103° C. The processing conditions are shown in Table 1. TABLE 1 ExampleNumber 1 2 3 4 5 6 7 8 Inv Inv Inv Inv Inv Comp Comp Comp Cast Layer (A)Thickness 245 245 245 245 245 245 245 245 (micron) Cast Layer (B)Thickness 30 30 30 30 30 30 30 30 (micron) Cast Layer (C) Thickness 4848 48 48 48 48 48 48 (micron) Average Bead Size in 2 2 2 2 2 2 2 2 LayerB (micron) Average Bead Size in 5 5 5 5 5 n/a n/a n/a Layer C (micron)Approximate Symmetric 2× 2.5× 3× 3.5× 4× 3× 3.5× 4× Stretching ExtentStretching Temperature 103 103 103 103 103 103 103 103 (degree C.)Stretched Layer (A) 61 39 27 20 15 27 20 15 Thickness (micron) StretchedLayer (B) 8 5 3 2 2 3 2 2 Thickness (micron) Stretched Layer (C) 12 8 54 3 n/a n/a n/a Thickness (micron) Percent Total 78 71.5 69 72.4 75 8384.7 85 Transmission at 500 nm Percent Diffuse 76 70.3 68 70.8 73 5463.2 67 Transmission at 500 nm Percent Spectral 1.6 1.1 1 1.7 2.6 2921.5 18 Transmission at 500 nm Percent Diffuse 26 31.4 33 30.9 28 1716.8 17 Reflection at 500 nm

[0112] As the data above clearly indicates, microvoided polymerdiffusers containing multiple layers provided much higher diffusetransmission efficiencies. Comparing examples #3 and #6 it can be seenthat with approximately the same thickness and frequency of bead, #3with layers with varying bead size and thus varying void size, had amuch higher diffuse transmission efficiency, 68% versus 54%. Examples #4compared to #8 and examples #5 compared to #9 illustrate this increasedefficiency as well. The total transmission is higher at 500 nm for thesingle layer diffusers, but the spectral component of the light is veryhigh as well. For a backlit application, this means that these diffusionfilms do not fully diffuse the light, the backlight can be seen, and thelight is not evened out across the display. For the same thickness, if 2or more layers are manufactured instead of a single layer, the diffusetransmission efficiency will be greater thus lead to more efficientdiffusing for the backlight of an LCD display.

[0113] From the data above, the general trend of the data is for diffusetransmission efficiency to be roughly inversely proportional to voidedlayer thickness, thus demonstrating that a thin, microvoided diffuserwith mcirovoided layers provides excellent light diffusion and yet has ahigh transmission rate, allowing LC devices to be brighter. A brighterLC device has significant commercial value in that a brighter imageallows for a reduction in battery power and better allows the LC deviceto be used in demanding outdoor sunlight conditions.

[0114] Further, because the example materials were constructed fromoriented polyester, the materials have a higher elastic modulus comparedto cast diffuser sheets. Because the example materials were oriented,the impact resistance was also improved compared to cast diffuser sheetsmaking the example materials more scratch resistant. Finally, theoriented polymer diffuser layers of the example allow for the voidedlayer to be thin and therefore cost efficient as the materials contentof the example materials is reduced compared to prior art materials.

[0115] While this example was primarily directed toward the use ofthermoplastic materials for LC devices, the materials of the inventionhave value in other diffusion applications such as back light display,imaging elements containing a diffusion layer, a diffuser for specularhome lighting and privacy screens.

[0116] The entire contents of the patents and other publicationsreferred to in this specification are incorporated herein by reference.

Parts List

[0117]2; Light guide

[0118]4; Reflection tape

[0119]6; Reflection tape

[0120]8; Reflection film

[0121]10; Reflection tape

[0122]12; Light diffuser

[0123]14; Brightness enhancement film

[0124]16; Polarization film

[0125]22; Polymer matrix containing small voids

[0126]24; Small air voids

[0127]26; Interface between voided layers of different size

[0128]28; Polymer matrix containing large voids

[0129]30; Large air voids

What is claimed is:
 1. A light diffuser comprising a polymeric filmwherein the film comprises a plurality of layers having void geometry inwhich the x/y/z size or frequency varies by at least 28% between atleast two layers.
 2. The light diffuser of claim 1 wherein the polymericfilm comprises two voided layers.
 3. The light diffuser of claim 1wherein the polymeric film contains at least two voided layers and atleast one non-voided layer.
 4. The light diffuser of claim 3 wherein thevoided and non-voided layers are integral.
 5. The light diffuser ofclaim 3 wherein the polymeric film the non-voided layer furthercomprises addenda.
 6. The light diffuser of claim 1 wherein thepolymeric film contains at least two voided layers that are separated bya non-voided layer.
 7. The light diffuser of claim 1 wherein the saidplurality of voided layers that vary in geometry or frequency improvethe diffuse light transmission efficiency compared to a single voidedlayer of the same thickness and either void geometry or frequency by atleast 10% at 500 nm.
 8. The light diffuser of claim 1 wherein microvoidshave a substantially circular cross-section in a plane perpendicular tothe direction of light travel.
 9. The light diffuser of claim 1 whereinthe x/y/z size or frequency of the voids vary by between 28% and 300%between at least two layers.
 10. The light diffuser of claim 1 whereinthe x/y/z size or frequency of the voids vary by at least 60% between atleast two layers.
 11. The light diffuser of claim 1 wherein the voidedlayers are arranged in order of increasing size of voids in reference tothe light passing through the film.
 12. The light diffuser of claim 1wherein the voided layers are arranged in order of decreasing size ofvoids in reference to the light passing through the film.
 13. The lightdiffuser of claim 1 wherein the voided layers are arranged in order ofincreasing frequency of voids in reference to the light passing throughthe film.
 14. The light diffuser of claim 1 wherein the voided layersare arranged in order of decreasing frequency of voids in reference tothe light passing through the film.
 15. The light diffuser of claim 1wherein the film contains at least one polymeric skin layer.
 16. Thelight diffuser of claim 1 wherein the difference in refractive indexbetween the thermoplastic polymeric material and the microvoids isgreater than 0.2.
 17. The light diffuser of claim 1 wherein saidmicrovoids are formed by organic microspheres.
 18. The light diffuser ofclaim 1 wherein the microvoids contain cross-linked polymer beads. 19.The light diffuser of claim 1 wherein the microvoids contain a gas. 20.The light diffuser of claim 1 wherein the elastic modulus of the lightdiffuser is greater than 500 MPa.
 21. The light diffuser of claim 1wherein said diffuse light transmission efficiency is greater than 80%at 500 nm.
 22. The light diffuser of claim 1 wherein said diffuse lighttransmission efficiency is greater than 87% at 500 nm.
 23. The lightdiffuser of claim 1 wherein said microvoids have a major axis diameterto minor axis diameter ratio of less than 2.0.
 24. The light diffuser ofclaim 1 wherein said microvoids have a major axis diameter to minor axisdiameter ratio of between 1.0 and 1.6.
 25. The light diffuser of claim 1wherein said thermoplastic layers contain greater than 4 index ofrefraction changes greater than 0.20 parallel to the direction of lighttravel.
 26. The light diffuser of claim 1 wherein said microvoids have aaverage volume of between 8 and 42 cubic micrometers over an area of 1cm².
 27. The light diffuser of claim 1 wherein the said light diffuserhas a thickness less than 250 micrometers.
 28. The light diffuser ofclaim 1 wherein said thermoplastic layer comprises polyolefin polymer.29. The light diffuser of claim 1 wherein said thermoplastic layercomprises polyester polymer.
 30. The light diffuser of claim 18 whereinsaid cross linked polymer beads have a mean particle size less than 2.0micrometers.
 31. The light diffuser of claim 18 wherein said crosslinked polymer beads have a mean particle size between 0.30 and 1.7micrometers.
 32. A back lighted imaging media comprising a light sourceand a polymeric film incorporating microvoids wherein the film comprisesa plurality of layers having void geometry in which the x/y/z size orfrequency varies by at least 28% between at least two layers.
 33. Anliquid crystal device comprising a light source and a polymeric filmincorporating microvoids wherein the film comprises a plurality oflayers having void geometry in which the x/y/z size or frequency variesby at least 28% between at least two layers.
 34. A liquid crystal devicecomponent comprising a light source and a polymeric film incorporatingmicrovoids wherein the film comprises a plurality of layers having voidgeometry in which the x/y/z size or frequency varies by at least 28%between at least two layers.